1. Field of the Invention
The present invention relates to a magnetic powder dispersed binder type carrier for electrophotography developers which carrier is used in developing an electrostatic latent image formed by an electrophotography method or electrostatic printing method, a two-component developer for electrophotography containing this carrier and a method for forming an image using this two-component type developer for electrophotography.
2. Description of the Prior Art
A method of developing an electrophotography is a method of developing by adhering toner particles present in a developer onto an electrostatic latent image formed on a photosensitive material. The developers used in this method are classified into a two-component developer composed of toner particles and carrier particles and a one-component developer composed of only toner particles.
Among the developing methods with these developers, as the developing method using such a two-component developer composed of toner particles and carrier particles, a cascade development method has been employed formerly but at present, a magnetic brush developing method using a magnet roller is mainly employed.
The carrier particles contained in the two-component developer are stirred with the toner particles in a developing box filled with the developer and thereby impart desired electric charge to the toner particles, and further act as a carrier substance for transporting the charged toner particles to the surface of a photosensitive material in order to form a toner image on the photosensitive material. The carrier particles remaining on a developing roller with magnetism return to the developing box from the developing roller again, and are mixed and stirred with new toner particles. In this way, the carrier particles are used repeatedly for a certain period of time.
Different from the one-component developer, the two-component developer has a function such that the carrier particles are mixed and stirred with the toner particles to charge the toner particles and further to transport them. Therefore, the two-component developer has good controllability in designing the developer. Accordingly, the two-component developer is suitable for a full color developing device in need of having high image quality and a first printing device in need of having reliability for keeping image and durability.
The two-component developer thus used needs to have image properties such as image density, fog, white spots, tone properties and resolving power which properties show the prescribed values from an initial stage and further, even if using the developer for a long period of time, are stably maintained without change. In order to maintain these properties stably, the carrier particles contained in the two-component developer need to have stable properties.
The carrier particles forming the two-component developer used conventionally are iron powder carriers such as iron powder which surface is covered with oxide and iron powder which surface is covered with a resin. The iron carriers have high magnetization and high conductivity so that they have merits capable of easily preparing image of good reproducibility in a solid part.
The iron powder, however, has a heavy own weight and too high magnetization. On this account, when the iron powder is stirred and mixed with toner particles in a developing box, fusion of the toner to the surface of the irons powder carrier, that is, toner-spent condition is apt to occur. Due to the occurrence of the toner-spent condition, the effective carrier surface area is decreased and thereby the triboelectric charging capability with the toner particles is apt to be lowered.
With regard to the resin-coated iron powder carrier, the surface resin is peeled off by stress received from use for a long period of time and thereby a core material (iron powder) having high conductivity and a low dielectric breakdown voltage is exposed to induce leak of electric charge occasionally. The leak of electric charge breaks electrostatic latent image formed on a photosensitive material and thereby brush traces and the like are generated in a solid part so that uniform image is hardly prepared and the durability is inferior. For the above reasons, iron carriers including the oxide coated iron powder and the resin coated iron powder have been scarcely used at the present state.
Recently, as shown in JP-A-59 (1984)-48774, in place of the iron carriers, resin-coated ferrite carriers having a light true specific gravity of about 5.0, prepared by using a ferrite having low magnetization as a core material which surface is coated with a resin have been used frequently and the life of developers is prolonged greatly.
However, recently, as office networking is progressing, copy machines have progressed from single functional copy machines to multi-functional machines, and further the service system has shifted from such a system that periodical maintenance, for example, replacement of developers or the like is conducted by service engineers under contact, to a maintenance free system. The demand for prolonging the lifetime of the developers has further arisen in the market.
In offices, full color images are recognized so that the demand for high image quality has been increased and the diameters of toner particles are decreased for attaining high resolution.
For coping with such demands, it is necessary to charge toners with desired electricity quickly. For the sake of necessity, the particle diameter of carriers has been shifted to be small and the specific surface area thereof has been large. When the whole particle size distribution is shifted to the direction that the particle size is diminished, scattering or adhesion of the carrier particles having a small particle diameter to a photosensitive material is easily induced and thereby fatal image defects such as white omission and the like are easily induced. Consequently, the problems of the two-component developer are more revealed. Accordingly, the carriers having a small particle diameter are required to be controlled so as to further narrow the width of the particle size distribution.
For solving the above problems, many magnetic powder dispersed binder type carriers obtainable by dispersing fine magnetic powder in a resin are proposed with the aim of reducing the weight of the carrier particles and prolonging the life of the developers.
For example, JP-A-Hei 5(1993)-40367 discloses a carrier for electrostatic latent image development obtainable by kneading a resin and a magnetic powder and then pulverizing and classifying. However, such a magnetic powder dispersed binder type carrier prepared by the pulverization method easily induces leak of electric charge because the magnetic powder is exposed on the surface in pulverization, and further electrical scattering of the carrier to a photosensitive material cannot be decreased and consequently the life of the carrier is short under the present conditions. The carrier, further, has inferior environmental stability in charging due to excessive exposure of the magnetic powder. Additionally, the shapes of the carrier particles after the pulverization and classification are not uniform so that the surface areas of the particles are different from each other in the carrier. Furthermore, the fluidity of the developer is inferior so that the charge imparting capability toward the toner is uneven. As a result, quick charging capability is not obtained and thereby high quality image is not obtained. The operation environment in the pulverization step is not good, and further the pulverization step brings productive deterioration with lowering of the yield because the particle size distribution of the resulting carrier particles is widened.
To cope with these problems, for obtaining a spherical magnetic powder dispersed binder type carrier without passing through pulverization and classification steps, JP-A-Hei 2(1990)-220068 discloses a magnetic carrier, which comprises complex particles of ferromagnetic fine particles and a cured phenol resin and is obtainable by reacting and curing a phenol and an aldehyde in an aqueous medium. JP-A-Hei 8(1996)-334931 discloses a binder type carrier obtainable by suspension polymerization of isocyanate, a phenol and an aldehyde with heating under stirring in the presence of a magnetic powder. JP-A-Sho 62(1987)-296156 discloses a carrier obtainable by melt kneading a polyolefin thermoplastic resin and a magnetic fine powder, followed by spraying, cooling and solidifying.
As described above, various binder resins capable of dispersing and combining fine magnetic powders are disclosed for the magnetic powder dispersed binder type carriers. The curing phenol resin, however, has a high critical surface tension so that toners are easily fused and thereby induce toner spent condition in the use thereof. Further, the curing phenol resin invites a lowering of charging and has a short life. Further, the curing phenol resin has a benzene ring, it has such problems that the environmental dependency is high and the image quality is largely changed under high temperature and high humidity or low temperature and low humidity conditions.
The thermoplastic resin easily induces image defects such as white spots and the like because it has low heat resistance and carrier particles scattered are molten and solidified on a fuser roller. The polyolefin resin originally has low charging capability toward toners with negative polarity and easily induces toner scattering or fog so that sufficient image quality cannot be obtained.
JP-A-Hei 5(1993)-100496 discloses a binder type carrier in which a magnetic powder is dispersed in resin particles, obtainable by suspension polymerization of a composition containing a polymerizable monomer such as styrenes and the like, a silicone compound having a group capable of reacting with the polymerizable monomer, a cross-linking agent and a magnetic powder, in an aqueous medium.
However, when a binder phase is formed by polymerizing the polymerizable monomer such as styrene, methyl methacrylate and like, and the silicone compound capable of reacting with the monomer in the presence of the cross-linking agent and the magnetic powder, a uniform copolymer cannot be obtained and a polymer composition is distributed because the reactivity of the polymerizable monomer such as styrenes etc and that of the silicone compound are different each other. Furthermore, these polymers have inferior compatibility and thereby induce phase separation. As a result, sufficient mechanical strength cannot be obtained and the charging properties of the carrier particles vary with the result that high quality image cannot be obtained. Additionally, unreacted materials having a low molecular weight are remained and thereby not only the environmental dependency, which is one of the carrier properties becomes worse but also the mechanical strength of the carrier particles is low, the magnetic powders are released and the carrier has inferior durability. When the content of the silicone compound is increased in order to improve the spent resistance, the above problems tend to be exhibited markedly as described in this publication. At the present state, for the demands of further prolonging the life of developers and improving the image quality by improving the resistance to spent toner, mar resistance and mechanical strength, sufficient carriers are not prepared.
JP-A-Hei 8(1996)-286428) discloses a developer for color electrophotography characterized in that a binder resin of magnetic matter dispersed carrier particles comprises a silicone resin as an essential component.
The magnetic matter dispersed carrier particles containing the silicone resin as an essential component wherein the curing reaction of the silicone resin is a molecular elimination-condensation reaction, as described in this publication, easily cause voids generated by evaporation of byproducts having a low molecular weight or internal cracks caused by the change in specific gravity (volume) at the condensation time. Recently, developing apparatuses have been desired to be downsized, however, the magnetic matter-dispersed carrier prepared by this publication cannot sustain increase of stirring stress in a downsized developing apparatus and further it is difficult to prevent deterioration of the charging properties caused by release of the magnetic powder. Furthermore, similar to JP-A-Hei 5(1993)-100496, in the case of mixing the silicone resin and other resins as a binder resin, the silicone resin has inferior compatibility with the other resins so that phase separation is easily caused in the inside of the binder resin phase and further it is difficult to prepare carrier particles having uniform charging properties and sufficient mechanical strength.
Furthermore, as described in the publication, the case of preparing a magnetic powder dispersed binder type resin carrier using, as a binder resin, silicone resin fine particles such as F200 and R900 manufactured by Dow Corning Toray Silicone Co., Ltd has the following many problems. Because of the properties of this resin, pulverization and classification steps are indispensable. Accordingly, as the carrier shape is un-uniform, the surface areas of the carrier particles are different from each other. Further, as the fluidity of the developer is inferior, the capability of imparting charging toward toners is un-uniform. Therefore, the capability of quick charging cannot be attained and also high quality image cannot be obtained.
The silicone resin or silicone modified resin as disclosed in this publication is a solid silicone resin already prepared or a varnish containing a solvent. For preparing a spherical magnetic powder dispersed binder type carrier without the steps of pulverization and classification, even when the preparation will be tried through a polymerization method or a production step of suspending in an aqueous medium and curing, it is difficult to incorporate these silicone resins in an originally solid state into the preparation process. Further, the silicone varnish including a solvent easily induces voids because a large amount of solvent components contained in the varnish evaporates in curing. Furthermore, the mechanical strength of the resulting particles lowers and also particles having a uniform particle diameter and a uniform shape cannot be prepared with dispersing and maintaining the magnetic powder uniformly. Therefore, this publication cannot solve the above problems essentially.
JP-A-Hei 2(1990)-272577 discloses a magnetic carrier prepared by dispersing magnetic powder and conductive fine particles in a binder resin formed from a silicone resin containing an organic tin compound. When the silicone resin is used as an essential component and the carrier is prepared with condensation reaction in the presence of an organic tin catalyst, large amounts of byproducts such as water, alcohol etc are generated. Due to the generation of the byproducts, voids are generated inside the carrier and simultaneously due to change in specific gravity (volume), cracks are easily generated to lower the strength of the carrier. Additionally, release of the magnetic powder is easily induced, the charged amount thereof is markedly decreased and sufficient durability cannot be attained. Therefore, the carrier is not a sufficient one.
When the carrier is prepared by condensation reaction using the silicone resin as an essential component and the organic tin catalyst, the organic tin compounds remains in an end product. The organic tin compounds are widely known as an endocrine disturbing chemical (environmental hormone) similar to formaldehyde. Taking the recent environmental problems into consideration, it is desired to avoid the use of such a substance in the preparation.
JP-A-Hei 10(1998)-39549 discloses a magnetic matter dispersed resin carrier which surface is covered with a resin composition at least containing a straight silicone resin and a coupling agent. The adhesion between the outer surface silicone resin layer and the magnetic powder dispersed binder resin layer as described in this publication is low so that during the use, the covered resin is easily released and the magnetic powder present in the exposed carrier core material surface is easily released. As a result, the charging and the resistance are changed to invite deterioration of the image quality.
It is an object of the invention to provide a carrier for electrophotography developers which is produced with slightly generating byproducts such as water, alcohol etc, without release of a magnetic powder, and has high mechanical strength, excellent durability and good environmental stability, and further can control toner spent condition, and has good fluidity and excellent capability of imparting charging to a toner.
It is another object of the invention to provide a two-component type developer containing the above carrier for electrophotography developers having the above properties.
It is a further object of the invention to provide a method for forming an image by using the above two-component developer by an alternating electric field.